Volvo: Volvo Trasmission Flush, transmission oil change, trasmission

Question
Volvo 2000 s70 with 69,000 miles; Was recommended to have the transmission oil change even though no problems with transmission, took it to a mechanic had the transmission oil change received car back with a transmission needing replacing. Transmission now slips on first and 3rd gear to a point that it does not drive safely. Mechanic says it is not his fault and that he warned me. The only warning he said was he needed to get Volvo oil and some additive other wise the transmission could go bad if he uses the wrong oil or no adaptive so he needed to charge an extra $40. What could have caused the failure?

Answer
Daniel,
First do you know what kind of oil your trans used?  Do you have a 5 speed trans? If it was synthetic and Dextron was used that would harm the trans. Could there be some fault codes in the system? Like Gear position sensor? When that goes bad the same thing tends to happen.